1. Field of the Invention
The present invention relates to a technology of controlling a bit rate of cells in a cell network, for example, an ATM network.
2. Description of the Related Art
An available bit rate (ABR) service has been introduced as a service based on a data exchange technology with an asynchronous transfer mode (ATM) method in an ATM forum and an ITU-T. The ABR service allows the network to avoid a network congestion by feeding back the network congestion information from a switching unit to a source terminal. That is, the ABR service is designed to efficiently operate a network, avoid the congestion in the network, and reduce the loss of cells by monitoring the condition of the network resources and altering the transmission rate of the source terminal through the switching unit. Described below is the detail of the ABR service.
FIG. 1 shows the rate control in the ABR service.
In FIG. 1, 101 and 102 are terminals. 103 and 104 are ATM switching units forming part of an ATM network. 107 is a main information cell (user cell). 108 is a forward (from a source terminal to a destination terminal) resource management cell (F-RM cell). 109 is a backward (from a destination terminal to a source terminal) resource management cell (B-RM cell).
Thus, the resource management (RM) cells are used in the communications through the ABR service to notify a terminal of the information about network resources.
The source terminal 101 sends the F-RM cell 108 each time N.sub.rm -1 main information cells 107 are transmitted. The F-RM cell 108 reaches the destination terminal 102 through the ATM switching units 103 and 104.
When the destination terminal 102 receives the main information cell 107 and the F-RM cell 108, it loops back the F-RM cell 108 and sends it as the B-RM cell 109 to the source terminal 101.
On the other hand, the ATM switching units 103 and 104 notify the source terminal 101 of the information from the network by writing the resource information (band width information and congestion information) in the switching unit to the RM cells 108 and 109 passing through the ATM switching units 103 and 104. When the source terminal 101 receives the RM cell to which the band width information, congestion information, etc. are written, it computes its own allowed cell rate (ACR) again according to the information, and performs communications at a rate of or lower than the ACR.
At the start of the communications, the source terminal 101 declares the maximum transmission rate, that is, the peak cell rate (PCR), and the minimum transmission rate, that is, the minimum cell rate (MCR), and negotiates with the network. The source terminal 101 cannot send an ATM cell (hereinafter referred to simply as a cell) at a rate higher than the PCR determined as a result of the negotiation. The ATM switching units 103 and 104 in the ATM network guarantee the source terminal 101 a rate higher than the MCR determined as a result of the negotiation. Therefore, the allowed cell rate ACR in the source terminal 101 alters within the range between the MCR and the PCR (MCR.ltoreq.ACR.ltoreq.PCR).
In these operations, the ATM network can avoid the congestion or recover from the congestion, and the terminals can transmit cells at a high transmission rate when the network resources are available.
The operations of the source terminal (ABR source terminal) 101 and the destination terminal (ABR destination terminal) 102 in the communications using the ABR service are standardized by the ATM Forum. Briefly described below are the important operations of these terminals.